One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition in which the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique for restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses, and in the field by emergency treatment personnel, e.g., paramedics.
Conventional external cardiac defibrillators first accumulate a high-energy electric charge on an energy storage capacitor. When a switching mechanism is closed, the stored energy is transferred to a patient in the form of a large current pulse. The current pulse is applied to the patient via a pair of electrodes positioned on the patient's chest. Different switching mechanisms may be used depending in part on whether the defibrillator applies a monophasic or multiphasic defibrillation pulse to the patient. A discharge control signal causes the switching mechanism to complete an electrical circuit between the storage capacitor and the defibrillator output terminals that are connected to the electrodes attached to the patient.
One prior art circuit that uses relatively expensive semiconductor switching elements in an output circuit to deliver biphasic defibrillation pulses is shown in FIG. 1A. FIGS. 1A and 1B are taken from prior art U.S. Pat. No. 5,083,562 to de Coriolis et al. FIG. 1A shows four relatively expensive semiconductor switching elements 40, 42, 44, and 46, which are used to deliver the energy from a large storage capacitor 39 to a patient's heart 18. As shown, when the switches 40 and 42 are closed, the energy flows from the capacitor 39 through the heart 18 in the downward direction, as indicated by arrow 68. To accomplish the second phase of the biphasic pulse, switches 40 and 42 are opened and the switches 44 and 46 are closed, thus causing the remaining energy from the capacitor 39 to flow through the heart 18 in the upward direction, as indicated by arrow 70.
The biphasic pulse that is produced by the circuit of FIG. 1A is shown in FIG. 1B. As can be seen in FIG. 1B, during the first phase of the biphasic pulse when the switches 40 and 42 are closed, the voltage on the storage capacitor 39 drops from V1 to V2. Then, when the switches 40 and 42 are opened and the switches 44 and 46 are closed, the energy storage capacitor 39 is essentially flipped over, causing the remaining voltage V2 on the storage capacitor to be essentially referenced as negative voltage. The negative voltage V2 then flows through the heart as the second phase of the biphasic pulse until the voltage level reaches V3, at which time switch 48 is closed to discharge the remaining energy from the storage capacitor 39.
FIGS. 2A and 2B are taken from prior art U.S. Pat. No. 5,468,254 to Hahn et al. FIG. 2A shows a similar circuit to that of FIG. 1A in that it also uses four semiconductor switches. In the circuit of FIG. 2A, two of the switches are shown to be SCRs (semiconductor- or silicon-controlled rectifiers) while the remaining two switches are shown merely to be electronic. Given the large voltages and currents used in defibrillators, the most commonly used semiconductor switching elements are SCRs and IGBTs (insulated gate bipolar transistors). FIG. 2B shows the biphasic pulse produced by the circuit of FIG. 2A.
Defibrillators such as those shown in FIGS. 1A and 2A, which apply a biphasic pulse and use semiconductor switching elements, are relatively new in the art of defibrillators. Older defibrillators usually applied only a monophasic pulse and used low-cost relays rather than the more expensive semiconductor switching elements. The reason the low-cost relays are generally thought to be incapable of use in the newer biphasic defibrillators is due to the fact that the low-cost relays are unable to be controlled with the precision required for biphasic defibrillation pulses. More specifically, when the switching elements of a relay are physically opened, energy tends to arc or spark across the relay if the voltages or currents are too high. High voltages and currents are especially present in external defibrillators that are designed to apply a defibrillation pulse to a patient externally through the patient's skin and chest (wherein more tissue causes greater resistance). This is in contrast to internal defibrillators that are surgically implanted into a patient so as to conduct the energy directly to the patient's heart tissue (wherein less tissue means less resistance). The arcing problems of relays do not generally occur with semiconductor switching elements that, unlike relays, do not have any moving metal parts and so do not have the same arcing or sparking problem.
The arcing phenomenon is especially problematic in biphasic defibrillators that, as described above, have to stop the flow of high energy in between the first and second phases of a biphasic pulse. If a simple conventional relay is used in a biphasic defibrillator to break the energy flow, the energy may arc across the relay. In addition to wearing out the relay, this arcing would drain the capacitor of its remaining energy that is supposed to be used for the second phase of the defibrillation pulse. Thus, the potentially life-saving second phase of the defibrillation pulse would be eliminated. Again, this is not an issue in monophasic defibrillators, because they only deliver one pulse that uses almost all of the energy of the storage capacitor. The monophasic defibrillators do not try to stop the high energy flow during the capacitor discharge when such an interruption is more likely to result in arcing or sparking.
The present invention is directed to providing an apparatus that overcomes the foregoing and other disadvantages. More specifically, the present invention is directed to providing an output circuit for a biphasic defibrillator that reduces the number of expensive semiconductor switches that are used.